In recent years, an OLED display has been attracting attention as a flat display panel. The OLED display can be driven by a DC voltage, and hence its drive circuit can be simplified. In addition, the OLED display has advantages such as having no viewing angle dependence unlike a liquid crystal display, being bright due to its self-luminescence, and having a rapid response speed. At present, the OLED display is mainly used in a small portable appliance, such, as a mobile phone, and is expected to be applied to an ultra thin screen television in future. It should be noted that the OLED display mainly uses a system in which an active element, such as a thin-film transistor (TFT), is arranged at each pixel for driving as is the case with the liquid crystal display.
The OLED display comprises, for example, two glass substrates, a negative electrode made of a metal or the like, an organic light-emitting layer, a positive electrode made of ITO or the like, and a bonding material. Hitherto, an organic resin-based bonding material, such as an epoxy resin having a low-temperature curing property or a UV curable resin, has been used as the bonding material. However, the organic resin-based bonding material cannot block penetration of gas completely. Thus, when the organic resin-based bonding material is used, air tightness inside the OLED display cannot be maintained. Owing to this, the organic light-emitting layer, which has low water resistance, is liable to degrade, resulting in such a failure that a display characteristic of the OLED display degrades time-dependently. In addition, the organic resin-based bonding material has an advantage of being able to bond glass substrates to each other at low temperature, but has low water resistance. Therefore, when the OLED display is used over a long period, reliability of the display is liable to deteriorate.
Meanwhile, a sealing material containing glass powder has excellent water resistance and is suitable for providing the air tightness inside the OLED display, as compared to the organic resin-based bonding material.
However, the glass powder generally has a softening temperature of 300° C. or more, and hence it has been difficult to apply the glass powder to the OLED display. Specifically, when the glass substrates are sealed with each other with the above-mentioned sealing material, it has been necessary to put the whole OLED display in an electric furnace and fire it at a temperature equal to or higher than the softening temperature of the glass powder, thereby softening and flowing the glass powder. However, the active element used in the OLED display only has heat resistance to a temperature of from about 120° C. to about 130° C., and hence, when the glass substrates are sealed with each other by this method, the active element is damaged by heat, resulting in degradation of a display characteristic of the OLED display. In addition, an organic light-emitting material is also poor in heat resistance, and hence, when the glass substrates are sealed with each other by this method, the organic light-emitting material is damaged by heat, resulting in degradation of a display characteristic of the OLED display.
In view of the circumstances mentioned above, laser sealing has been investigated in recent years as a method of sealing the OLED display. The laser sealing can locally heat only the sites that should be sealed, and hence the glass substrates can be sealed with each other while thermal degradation of the active element and the like is prevented.
As an example of the laser sealing, in Patent Literatures 1 and 2, there is a disclosure of laser sealing of glass substrates of a field emission display.